Analog Reference Voltages

The AVR's A/D converter needs a reference voltage to set its converting range. STK600 can supply two of these voltages, AREF0 and AREF1.

For all tinyAVR and megaAVR devices, only the AREF0 is connected trough the routing card, whilst for xmegaAVR devices, AREF0 is connected to ArefA and AREF1 is connected to ArefB.

For xmega devices the AREF pins is also general purpose I/O pins. Hence, to use the pins as GPIO the Aref jumpers must be removed.

Note: For routingcard "STK600-RC100X-13" revision A and revision B the AREF1 is connected to PA1. These cards are marked "A0607.3.1213.A" and "A0607.3.1213.B"

If the AREF0/AREF1 jumpers are mounted, the on-board Analog Reference Voltage sources are connected to the target AVR’s AREF pins. The On-board Analog Reference Voltages can be adjusted from the PC software in the range 0 to 5.5V, but not above VTARGET. The resolution and accuracy is 10mV.

The AREF0 and AREF1 generated voltages can also be connected to the analog comparator.

The target AVR’s AREF signals are accessible on the AUX header. The figure below shows the connection of the AREF signals, the target section and the AREF sources.

Using external voltage reference

When the AREF0/AREF1 jumper is disconnected, ADC reference voltage can be supplied from an external source, by connecting to the AREF0/AREF pins on the AUX header.

When using external an source for AREF, the user must control that VTARGET is at a higher voltage level than AREF. This can be controlled easily by reading the VTG value from AVR Studio before setting AREF.

NOTE: The AREF0 and AREF1 voltage which are visible in the PC software are the STK600 generated voltages. Externally applied AREF voltages cannot be read from AVRStudio.

Using the internal voltage reference

If the AVR's internal voltage reference is used, the AREF0/AREF1 jumper must be removed.

Using Aref as analog input

AREF0 and AREF1 can also be used as analog inputs to any of the ADC channels.

For tiny and mega devices, where only AREF0 is connected to the device via the routing card, AREF1 can be connected to a ADC channel by connecting a cable from AREF1 on the AUX header and to the port pin header corresponding to the adc channel. (check the device datasheet for which port pin header to connect to).

If the AVR's internal voltage reference, or an external voltage reference is used, the AREF0/AREF1 jumper must be removed. If this is the case, AREF0 or AREF1 can be used as analog inputs by connecting a cable from pin 1 on the AREF0/AREF1 header to the port pin header corresponding to the adc channel.

AREF decoupling capacitor

The routing card has a decoupling capacitor on AREF. This is marked with silkprint on the PCB. For some AVR devices the AREF pin is on a pin which also is part of the high-voltage programming interface. On these routing cards the aref capacitor is not mounted, as it would make it impossible to use the high-voltage programming interface. A capacitor can be soldered to achieve better noise performance. A typical value is 10nF. The footprint for the capacitor is SMD size 0603.

Short circuit protection

The internal AREF voltage generators have a short circuit protection. If the STK600 measures the AREF0/AREF1 to be 0.3V or more below the setpoint, AREF will be shut off. When this happens, the status LED will blink red. The AREF0 and AREF1 will also be shut down by the Master MCU if a short circuit is detected on VTarget (in addition to shutting down VTarget). In this case, the status LED will blink red.